(1) Field of the Invention:
This invention relates to a resin-made optical fiber having excellent heat resistance.
(2) Description of the Prior Art:
It has been known over many years to use light-carrying optical fibers, each of which is composed of a core and a cladding on the outer wall of the core, as information transmitting means. Both glass-made and resin-made optical fibers are now actually employed. Although resin-made optical fibers are somewhat inferior in light-carrying capacity to their glass-made counterparts at the present time, they have practically advantageous merits such that they can be connected together rather easily, have light weights and superb flexibility, and can be fabricated at relatively low costs. For these merits, resin-made optical fibers have recently been finding utility in various fields.
As one of conventional fabrication processes of such resin-made optical fibers, a method has been known in which a highly-transparent and optically amorphous polymer or copolymer such as polymethyl methacrylate, polystyrene, polycyclohexyl methacrylate or polyphenyl methacrylate is prepared at first, the polymer or copolymer is melt molded to form a fibrous member by an extruder or the like, and then the fibrous member is covered as a core with a cladding by dipping technique, coextruding technique or the like. Specifically, it is disclosed, for example, in Japanese Patent Publication No. 42261/1978 and 42260/1978 that a monomer capable of providing a polymethyl acrylate polymer or copolymer having excellent transparency, mechanical properties, weatherability, etc., is used by way of example, impurities are removed from the monomer to avoid any detrimental effects to the light-carrying capacity, the thus-purified monomer is polymerized by the continuous bulk polymerization process to obtain a polymer, and the polymer is thereafter molten and molded to obtain a fiber useful as a core.
In conventional fabrication processes such as those described above, fibrous members useful as cores are fabricated by melt molding. Corollary to this, their materials, namely, polymers must have excellent melt moldability. For this reason, the provision of polymers with improved melt moldability has been the subject of a great deal of work, including the addition of various chain transfer agents or the like upon polymerization with a view toward reducing the molecular weights of polymers to be obtained finally and hence lowering their melt viscosities.
Such prior art processes however preclude the use of resin materials the melt molding of which is not feasible, for example, resin materials exhibiting poor stability at high temperatures to which they are exposed upon their melt molding, those having large molecular weights and hence high melt viscosities, crosslinked polymers the melting of which is difficult, etc. Reflecting today's wide-spread adoption of resin-made optical fibers owing to the diversification of the information industry, a variety of properties are now required for resin-made optical fibers. This had led, for example, to the demand for resin-made optical fibers having high heat distortion temperatures so as to possess sufficient heat resistance at high temperatures or resin-made optical fibers having sufficient flexibility even at high temperatures.
As an optical fiber excellent in both heat resistance and environmental resistance, European Patent Specification No. 153514 discloses an optical fiber making use of polysiloxane, which is an inorganic thermosetting resin, as a core material. In this optical fiber, the core-forming polysiloxane is inorganic in nature. Its bonding to an organic polymer, which forms the cladding of the optical fiber, is hence not sufficient. The optical fiber is accompanied by another disadvantage that the refractive index of the polysiloxane is small and a significant limitation is thus imposed on the selection of materials usable for the formation of the associated cladding.